1. Field of the Invention
This invention relates to a magnetic recording/reproducing apparatus used for an auxiliary recording apparatus of a computer, or the like. More particularly, it relates to a recording equalizer for correcting an interference between adjacent codes that occurs when magnetic information is recorded, and a magnetic recording/reproducing apparatus using the recording equalizer.
2. Description of the Related Art
Demands for a higher density and a higher memory capacity of magnetic disk apparatuses used as an external storage apparatus of a computer have become stronger and stronger with the progress of so-called "information society". The recording density of the magnetic disk apparatuses has become higher at present at an annual ratio of 40 to 60%. Such a rapid progress of the high density has been supported not only by the development of low noise media but also by the introduction of new technologies such as an MR (Magnetoresistive) head and a PRML (Partial Response Maximum Likelihood) system.
In comparison with a conventional 1-7/PD (Peak Detection) system, the PRMl system records information at a magnetization inversion density (FCI: flux changes per inch) of 1.5 times on a recording medium in the case of the same linear recording density (BPI: bits per inch). Therefore, various counter-measures, which have not been necessary in the past, must be taken in order to cope with various phenomena that occur at a high magnetization inversion density and in a high frequency range. One of such phenomena is a non-linear phenomenon at the time of recording represented by an NLTS (Non-Linear Transition Shift). In the PRML system, in particular, linearity of a recording/reproducing channel is required so as to fully exploit its performance.
In the PRML channel, judgement is done by using an amplitude value obtained by sampling a reproduction waveform and for this reason, sampling at a correct timing and equalization to a correct waveform position are essentially necessary. To achieve this object, the magnetization inversion position to be written by a magnetic head to a recording medium must be controlled correctly. If any demagnetizing field exists inside the medium, however, a head magnetic field is modulated and deviation occurs in the magnetization inversion position to be written. The NLS is the phenomenon in which this magnetization inversion position shifts from the inversion position that is expected from the timing of inversion of a recording current. The inversion of the recording current occurs when recording bits are "1".
Among the influences of the NLTS, the influence from one-previous bit is the greatest, and the magnetization inversion position to be written (the magnetization inversion position expected from the inversion timing of the head magnetic field) is shifted in a direction opposite to a travelling direction of a recording head as viewed from the recording medium. To correct such a shift of the recording position, a control method of a magnetization inversion position, which deviates the inversion timing of the head magnetic field in accordance with a recording pattern and controls the magnetization inversion position to a correct position on a medium (precompensation), has been employed in the past.
One of the methods of precompensating for the NLTS delays the inversion timing by a time corresponding to the shift described above from a timing of a basic clock by considering the magnetic field resulting from the magnetization transition occurred at the one-previous bit and further the two-previous bit. This method is described in IEEE Transactions on Magnetics Vol. 26, No. 5, September 1990, p2298.
When this method, which executes precompensation of the NLTS by delaying the inversion timing, is applied to a magnetic recording apparatus for achieving a recording density of at least 1.5 giga-bit per square inch, however, the magnetization inversion position gets disturbed and an erroneous operation occurs.
On the other hand, JP-A-8-96312 and JP-A-8-203006 describe recording/reproducing apparatus performing the precompensation.
The former describes a circuit for correcting amplitude characteristics and phase characteristics of a recording current and the latter describes a controller of an amplification gain of read data. Nonetheless, both do not consider the influence of the magnetization transition of the following bits of a recording bit.